The protozoan parasite Entamoeba histolytica causes amebiasis, which remains a major health problem in much of the world, and is considered a potential biological weapon. The two main forms of disease, amebic colitis and amebic liver abscess arise from complex interactions between the host and parasite. In the previous funding period, we focused primarily on the host, and established new paradigms about the role of inflammation and apoptosis in amebic disease. In this renewal we will return our focus to the parasite, and will look at what we have termed the amebic virulence program: E. histolytica molecules that must be expressed or suppressed to allow amebic invasion and survival within host tissues. We will define the virulence program through a transcriptional analysis, and ask how the virulence repertoire differs between amebic trophozoites in culture, those that have invaded into the colonic mucosa, and trophozoites within the liver. We will attempt to identify the requisite components of this response, and determine whether blocking their expression can alter the course of amebic colitis or amebic liver abscess. We will test the hypothesis that host macromolecules serve as triggers for the activation of the amebic virulence program, and test candidate amebic molecules for their role in recognizing host molecules and initiating the amebic virulence program: We will then determine whether blocking host macromolecule recognition, or signaling by amebic receptors, alters the course of amebic colitis. The above studies look at virulence in the context of amebic response to the host. We will also investigate the inherent virulence repertoire of E. histolytica HM1:IMSS by looking at differences at the transcriptional level between HM1 :IMSS and the reduced virulence E. histolytica Rahman. When complete, these studies should provide new insights into the biology of E. histolytica, will further our understanding of the pathophysiology of amebic colitis and amebic liver abscess, and will identify new virulence factors that may play unique roles in amebic colitis or amebic liver abscess and could be targets for new vaccines or therapeutics. [unreadable] [unreadable] [unreadable]